Subject Commit in execute block?

I am using Firebird 3 and I need to write one execute block statement which makes several alter table and update table commands, like:

execute statement 'alter table accounts add balance double precision'
execute statement 'update accounts set balance=previous_balance+10'

The problem is that commit is required after alter table (before the new field can be used in update command). The question - how can I issue some kind of commit inside execute block?

I am required to write multiple commands in one block, so, I can not separate statements.