I assume you've verified that all the ducts are connected correctly, ie
that none has fallen half apart from tape drying out, etc. Also make
sure any balance controls on the ducts going to the warm room are fully
open.

In addition to adding a return, which should help, there are several
other alternatives. There are booster blowers available that can be
inserted into a supply duct going to the area that is too hot. There
are also booster fans that can be placed over the vents. This type
senses when the air flow starts and turns itself on. I've tried the
first type and it definitely helped, though it's not fully effective.
Finally, you might want to consider adding a window unit to the room.
Of all the solutions, that one is guaranteed to work.

Usually this problem is the result of incompetent installers who didn't
run enough duct or return capacity to the farthest areas. And as
you've figured out, completely fixing it later isn't easy.