Filterless pipes will usually not take a filter.
The filters are placed in the shank or stem and it must be drilled out in order to do so. So by design, if there's no space for a filter, it ain't goin' in.
There are several types of filters... most common are the 9mm and 6mm filters. They are filled with either activated carbon/charcoal, meerschaum bits, or can just be balsa. Then you have the long skinny ones like Brigham's Maple 'Distillators' and the ones that go into MM cobs (Medico if I recall correctly, I don't use them).
Personally, I like the slimmer shanks on some non-filtered pipes that would just never be able to fit a filter. I smoke both filtered and non-filtered pipes. I find that filtering does dull the first part of a bowl on some blends.
There are some novel ways of putting a folded up pipe cleaner or some rolled up paper in your stem etc which works on some pipes.