You can buy standard Ha filters that screw into the front of your T-Adaptor. This is easy for all makes of camera as they all need a T-Adaptor to fit a scope focuser. Clip-in filters work but might be hard to find (or expensive) for the Pentax.
Concerning exposure for Ha, a typical DSLR has fewer red detecting pixels and the internal cut filters reduce the amount of Ha spectrum.
https://en.m.wikipedia.org/wiki/Bayer_filter
So yes you need longer exposures to capture Ha, and your green and blue pixels will get nothing through an Ha filter. The reason folks modify DSLRs, by removing the internal blue cast filter, is that it allows about 4x to 5x as much Ha capture.
So for an Ha image sub exposure i would suggest you start with 5x the amount of time required for a standard exposure. You can start out on targets that are great sources of Ha like Eta Carinae, and the Lagoon Nebula, and experiment with the sub lengths required for your camera through the Ha filter. Obviously, the longer exposures require good mount tracking and guiding.