How can I keep a part of a filename for all additional screenshots?

I'd like to take a screenshot, save it with a name I enter in "save as" [which has a counter # at the end], and then have shutter KEEP the part of the name I entered for all additional screenshots, updating only the counter for each one. How can I do that?

Example:

Some net buy_001.jpg [I entered the "Some net buy" part, replacing what was there before.]

Additional screenshots would then automatically be [with no need to add/change anything]:
Some net buy_002.jpg
Some net buy_003.jpg
Some net buy_004.jpg
......