If this is a contractual allowance above statutory minima then I can see no reason it cannot be varied, if that's what both parties agree to do. In effect you will simply be paying for extra days/hours of absence during the lock-down instead of later, when return to work is possible.

....And welcome to the "Community". 

Welcome to the forums Sarah
I see no problem with doing this provided you stay at or above the minimum statutory level.
Don't forget to think about / check whether these payments will be part of pensionable salary, or count for life insurance purposes.....
Remember also that if these amounts take the person over the £2,500 cap, 80% of the excess cannot be reclaimed.....

Finally, do undertake some cost simulations with your finance team - to make sure that putting this in place will not create cashflow difficulties in the current context.

Nice initiative!