Do you mean making signs out of paper (for example) and then gluing them to buildings? Sure, it's possible. I use Powerpoint and sometimes Word to create signs and print them out on a good quality ink jet printer. The nice thing about Powerpoint is that you can measure the size you need and then create a box of almost exactly that size right on the slide. (Zooming in to about 200% doesn't hurt either!)
I print on standard office copier /printer paper I like to make a fairly thick black border around the sign in order to disguise non-straight cutting. I generally mount the signs with white glue or "school glue" applied in as thin a coat as I can manage. When it makes sense, I will press the sign into the building to try to make the pattern of the underlying wall come through; although in N Scale that usually isn't that noticable.
Here are some examples:
Billboard on the Roebson Process Company. The billboard is from Athearn and the sign is pasted right over the State Stamp that was there. Good deals on these billboards can be found here and there.
Sign facing the camera on the Acme Fine Loophole Company. Note that the sign along the long wall was done with Woodland Scenics Transfers, one letter at a time, since I cannot print white effectively!
The red and yellow signs on Otto Stock's (get it?) Farm Supplies, including the "Entrance" over the door. I think that "Entrance" is in about 4 point type. The logos are MicroScale decals.
The billboard of Lucy and Snoopy on the building is from a billboard photo that I found on line... another source for good signs if you don't want to create your own. Billboard is from Athearn again without the supporting structure.
One of my favorites, and if you know your Looney Tunes, you know exactly where the billboard advertising "Friz" comes from. The AutoLite sign is a decal from MicroScale. I think the paper sign compares favorably!
