First and foremost, I believe that team and added value is everything for an early stage startup. Every founding member should be an indispensable asset to your business.

With regards to bringing in other experts as co-founders, ask yourself why this particular accelerator would make such a demand. Can you think of a reason why they would not reassured by your proposal to bring in these experts as hires rather than co-founders? Have you validated your initial approach with people other than angels (e.g. successful founders of hard-ware based startups)?

Lastly, ask yourself what your company has to gain by going forward with this particular accelerator. Do you feel they represent a unique opportunity that you could not find elsewhere? If so : see if there are ways to address their concerns in ways that that would be acceptable to you (e.g. appropriate vesting schemes that would protect your interests).