pillar? What do you mean? Pillar is wrong here. Do you possibly mean principles? by-laws? tenets? policies?

From the words that you suggest, "principles" comes relatively close. What I mean with "pillar" is that being volunteer-based was one of few basic foundations/pillars/principles on which the organization in question was founded. It is a core characteristic of the organization that cannot be changed because the founder founded the organization exactly because of a small number of "pillars" that he believes in. Just as democracy is a pillar of Western society, being volunteer-based is a pillar of this organization.

Why can't I use "pillar" in this context?

You can call a person a pillar of society, but you can't use the word for something abstract, IMHO

related discussion: die tragende Säule unserer Gsellschaft - the... - #9

pil·lar (plr)
n.
1.
a. A slender, freestanding, vertical support; a column.
b. Such a structure or one similar to it used for decoration.
2. One who occupies a central or responsible position: a pillar of the state.
http://www.thefreedictionary.com/pillar

Tilman, would you use Säule or Stütze in the same sentence in German?

Perhaps you could simply say this:
I have been involved in the club's management for many years, but have never worked for it full-time as it is fundamentally a voluntary organization.

In the overwhelming majority of contexts, the word "club" automatically implies a voluntary organization in any case. It might be more succinct to say:
I have been involved in the club's management for many years on a voluntary basis.

Thanks for your ideas! Couple of points:

- I do want to start the sentence with in spite of because it fits the context very well.
- which is in accordance with vs. as it is: Steve UK, I do like your sentence a lot more than mine, but my concern is that as it is a voluntary organization requires every volunteer not to work full time more so than my original sentence. However, the following sentence in the text that I am writing is that in THIS case I HAVE worked full time (which is therefore NOT in accordance with the principles but still of course allowed). Hence, I think that which is in accordance with the organization's principle that it is fundamentally a voluntary organization fits better than as it is fundamentally a voluntary organization. I might be wrong, though...

So, merging all the feedback, I would now write the sentence in the following way:

In spite of having been involved in the club’s management for years, I have never before worked for it full-time, which is in accordance with the organization’s principle that it is fundamentally a voluntary organization. (And in case you are interested how the text more or less continues: [This project] gave me the rare chance to spend more than just a few hours per day to work for the club. It gave me the possibility to focus on projects which we had been planning for years but for which we never had time due to the day-to-day workload. These projects I shall discuss in the next chapter of this report.)

I think that sounds already much better than my first version :) So, thanks for your help!

Hi Tilman. Glad we could help. Having read your comments, I still think you should shorten the end part. Your version means nothing different, and sounds a bit verbose. So:

In spite of having been involved in the club’s management for years, I have never before worked for it full-time, as it is fundamentally a voluntary organization.

I know you're not asking for corrections to the rest, but here are a couple of points anyway:
... a few hours a day working for the club. It gave me the opportunity to focus ...

Steve, thank you very much! As you say that the meaning is the same, I gladly take your version! Thanks also for the corrections to the rest of the text :) Your help is very much appreciated.