If the Logo is already in gerber format then you just may add it with a text editor.
Not a nice job to do, because you need to take care of the X and Y offset of the logo.
I assume with the professional version of GraphiCode´s GC-Prevue you can do this with a graphical tool. WYSIWYG.
There may be other gerber editors that can do this.
If you manage to scale and offset the logo gerber correctly and assign unique apertures, you can even merge it into your existing gerber file with a text editor.
I personally use GC-PrevuePlus for gerber editing, panel generation and similar operations. But there are many other tools that can be used, including some free tools as far as I'm aware of.
I used to be able to do something similar using GC-Prevue alone.
I was lucky that my fab house also had GC-CAM (the front end editing version) so I only had to import the logo etc. into a new layer, offset it to it's desired location then save it all as a GWK file (Graphicodes native format) and send that.
Later when I got editing software it was even easier.
There are freeware Gerber programs that will export also, so if you can import, combine layers and export then its easy. (sorry cannot remember any names)