Add a file to existing changelist
Solution 1:
In general you can use:
p4 reopen -c CLN FILE ...
to move open files into the specified changelist (regardless of whether they were open for edit, add, etc.).
CLN may be default to dissociate the specified files from any numbered changelist.
Solution 2:
You can say:
p4 edit -c changelist# file ...
where changelist# refers to the pending changelist.