contact the manufacturer of the machine with the attatchment specs and theywill re rate the machine and send you an updated data plate with the new load charts (they have to by law but i do not believe its free at least here in the us that is the case)
if its a big name machine im sure they will help u (toyota mitsu etc)

It's perfectly okay to use a turret head attachment with a counter-balance truck if you comply with the manufacturers lift ratings(attachment and truck). The attachment manufacturers should be able to help you with guideline figures but these are purely guidelines. On a 3 ton counter balance you should be able to work about 750Kg on a turret head safely..but don't take my word for it as the truck type and tyre type etc. all need to be taken into account.