If you're willing to accept and install surface wiring you might as well fully embrace it. Don't connect to the existing receptacle with a plug; instead add an "extension box" at that existing receptacle and run the surface conduit directly into that to make the connection permanent.
As for the proposed UF-B cable -- don't use that; it'll be unnecessarily difficult to work with. The location is dry so NM-B cable is an option (but don't use that, either). Pulling UF-B through conduit would seem impossible; pulling NM-B through conduit is still difficult, and either would require oversize conduit. Because you're going to install conduit the wiring can be even easier: use loose THHN/THWN/etc conductors. You'll be able to use 1/2" conduit and those conductors will practically pull themselves through it.
The 14/2 size is likely wrong also: if you've confirmed that the existing receptacle is truly on a 15 amp circuit then 14 gauge is OK, but circuits in a garage are often 20 amp. In that case you'll need 12 gauge conductors.
The conduit could be PVC as pictured, EMT, or a flat type designed for surface wiring. Wiremold is a brand name of one such kind of "surface raceway;" there are other brands too.
Here's a photo of a project somebody did with the Wiremold products. One of the outlets was existing and was flush to the wall surface. They've added an extension (in this case the "Wiremold surface conduit starter box") to get up onto the surface of the wall, then conduit/raceway and a surface box with another outlet. (Kevin Standlee, "New Outlet")
If you don't like or can't easily obtain the Wiremold products something similar might be done with cast aluminum or drawn steel parts. A steel 2-gang box was easy enough to find, but I've come up empty in my search for a steel 2-gang extension box (1-gang handy box extension, 1-gang to 2-gang adapter, and 4-11/16" extension are easy to find). I'm thinking it could be achieved with a 4-square extension box with a 2-gang device ring on both sides of the extension.