gnus-move-group-prefix-function

gnus-move-group-prefix-function is a variable defined in `gnus-sum.el'.
Its value is

gnus-group-real-prefix

• This variable may be risky if used as a file-local variable.

Documentation:
Function used to compute default prefix for article move/copy/etc prompts.
The function should take one argument, a group name, and return a
string with the suggested prefix.

You can customize this variable.