Delimitation price point/comma

Hello there.
Please, I need help in the following situation:
I have created a product feed. One of the marketplaces I made this feed for uses “.” the point for price delimitation, and the feed is created with commas. (the other feeds for marketplaces are fine)
I tried to change with regular help, if it contains “.” then “,” and to replace “.” with”,”, but it doesn’t work.
How can I do this, just for one of the feeds?
Thank you.