Note that in its list of modules (
Supported Cameras - DxO) DxO says that the HD-DA 55-300mm f4.5-6.3 PLM is now supported from PhotoLab 3.0.0. You don't need the update to the latest build of PL4 to install the modules for the PLM - I downloaded and installed them (for the K-3, KP and K-S2) in the previous build and they work!
In fact there are modules even for cameras that don't support KAF4, such as the K10D, K-7 and K-5. If you are using the 55-300 PLM with one of those, shooting wide open with manual focus, PL4 has you covered!
That leaves a question about versions older than PL3. I have PL2 on an old laptop, so I tried going through the Optics Module installer in that program. The PLM module is not available.
I then tried copying the modules for the PLM (K-3, KP and K-S2) which I had downloaded in PL4 and putting them in the modules folder for PL2. When I reopened PL2, it listed the modules for those cameras and the PLM amongst the installed modules, but when I opened some PLM images taken with the KP, they were shown as taken with an unsupported lens.
So if you have Optics Pro, or PL1 or PL2, it appears you are out of luck, unless someone can work out a hack.