It should be.... but how are you achieving macro? Tubes? Bellows? The big problem with the 55-300 is going to be light loss. You lose a stop on a slow lens. I'm not familiar with the lens, and I can't recall hearing that it had a macro function.
As per the following description
HD PENTAX-DA 55-300mmF4.5-6.3ED PLM WR RE / Telephoto Lenses / K-mount Lenses / Lenses / Products | RICOH IMAGING
maximum magnification it .3x. A macro lens like the 50 macro goes 1:1 or 1x. The 55-300 is only slightly better than the 16-85 or 18-135, and no where near lenses like the Sigma 70-300 with 1:2 (.5x) magnification. There is no TC that works with a Sigma 70-300, but you get as much magnification without the TC than the Pentax would with it, as long as you can handle the 7 foot minimum focusing distance, which is actually good for insects as you're far enough away not to disturb them.