If you can describe geometrically where you want the SRAFs, you could try OPCSBAR, which doesn't use models. The instructions for it are in the Calibre OPCsbar User's and Reference Manual. (calbr_sbar_useref.pdf in your Calibre installation)
Alternatively, it isn't that hard to turn models into a litho model; just package them in a directory with a Lithomodel file. The format is pretty basic. If you have a WORKbench license, there's a built-in lithomodel wizard to turn existing models into one. (Searching on "wizard" in the WORKbench manual should land you in the right chapter.) If you don't have WORKbench, you can do it manually.
1) Create a directory that is accessible from anywhere you are running Calibre.
2) In the directory, create a file named Lithomodel (no extension).
3) For just a resist model and optical model (assuming single exposure/not double patterning):
version 1
resist <filename>
mask 0 {
background <transmission>
mask_layer 0
optical <filename>
}
4) Replace <filename> with the name of the resist/optical model. Ideally, you copy them to the directory with the Lithomodel file, but relative and absolute paths also work; you just can't count on being able to copy the directory around where you need it.
5) For <transmission>, use whatever is on the "background" line of your setup file. (I'm assuming you have one of those.)
There are a bunch of other parameters you can specify (see "Litho Model Format" in the OPCverify manual), but it sounds like you have a simple case.