Under S25 of Hindu Marriage ACt, you can claim alimony (compensation) while submitting the petition itself as additional prayer or after the successful closure of case. Please be noted that if you are earning, the alimony will be inversely proportional to % of your income w.r.t. your husband's income.
During the pendency of your case also you can claim maintenance for your livelihood and also your lawyer expenses under S24 of Hindu Marriage Act, provided you are not sufficiently earning and you are not staying together with your hubby.
If he is really impotent, you need not put any unnecessary false allegations or unnecessary false cases as the impotency ground itself is more powerful than any other ground for break-up i.e. divorce/nullity and also for Alimony (compensation).