it would help to post the specific file you are dealing with. A lot of us use the "outline stroke" option in the flyout choices for "path" (under the "Object" menu)
You asked about "putting on an outline", but you said someone sent you the atrwork, so I'm guessing the outline already shows up in a preview of their artwork, but it's done with a stroke, so that the vector path for it isn't showing up.
I'm guessing that, because I deal with that on at least 80-90% of all the client provided artwork in the last 10 years.
You also said it's complicated, and I know all about complicated vector files that are built only to look good in preview mode, or in print. I just spent 3 hours fixing this top file into the one below, so I could cut sandblast mask. I used "outline stroke" a lot, but also half the tools in the "pathfinder" menu, ALL the concentration and patience I could muster, (and a few choice "designer" epithet's) to get me through this one:
Some of that time was increased because I had to make some decisions between what would be raised and what would be blasted. It was drawn as a 3 color job, so it wasn't an obvious choice in some cases... but lots of making copies of multiple contours & then trimming one with the other, pasting both back, them trimming the other with the one..
I'll tell you, on the really complex ones, I used to just get a good looking black line art on the screen in "preview mode (with all the un-resolved outline overlaps and strokes still hidden away) and I'd print it out, scan the print, and run the scan through streamline. It's a 5 minute way to do the 3 hour job described here... but for $200 banners instead of $2000 sandblasted signs... it's a trick worth remembering sometimes.