Not sure where you're finding trouble, but the following thoughts ...

(1) Put your code into appropriate modules in the existing worksheets, and name
those modules.

(2) Open your target worksheet as well, then simply drag the modules into it
from within the VB editor. It will copy, rather than move, and the unique
module names will allow all the code to co-exist.

(3) You will need to avoid naming conflicts for subroutines and functions. I
prefer "dot" notation for VBA, where I use both the module name and the
subroutine/function name for all except "common subroutines". I use meaningful
and unique names for the modules, and much more generic names for the
subroutines / functions. E.g. DatabaseAccess.Initialise would start up a
database access module - not a non-descript module name and a subroutine name of
DatabaseAccessInitialise. This is closer to the modularising offered by other
languages.

Not sure what you mean by a tool  to edit  VBA files. Why not just edit them from
within Excel?