[Typo3] adding a sheet to existing flexform ds?

[Davide Principi]

I'd like to add a sheet to an exsisting flexform ds, to "extend an 
extension".

Perhaps it's possible to read the original xml file, add the sheet 
reference and override the original xml, in ext_tables.php

But it seems slow, as ext_tables.php (or the temp_cache*) are executed 
on every request.

Is there another way?

Thanks,
--Davide Principi (dabba)