Can I convert to tubless ?

IIRC, DT does conversion kits for its own rims, but you could do a "ghetto" (use search function) conversion which would be cheaper.
Its upto you which you feel more comfortable with.

It works with alot of tires (both wire and folding, 1 and 2+ ply, etc) - but those with weak beads (Stans/Notubes recommended list) will "roll off" easily, and those with thin sidewalls wont last too well. Some tires will leak sealent through the sidewalls and be near impossible to get sealed - search for conversions on specific tires (the most common tires will likely have posts about them).
Aslong as the bead is good and the sidewalls arent scary thin, you should be fine with a conversion.

Ideally, a UST tire would be best (brand is irrelevant, so you could ignore the notubes list here) because they're made specifically for tubeless and have stronger beads and suitable sidewalls.

Tubeless Ready are another option if your concerned with bead strength. These tires are designed to run tubeless but require sealant to stay airtight. Models such as Hutchinson Tubeless Ready, Bontrager TLR, Specialized 2Bliss, Stan's NoTube, and Geax TNT models are all tubeless ready and will work with DT, Stans, or Ghetto tubeless systems.

As a follow up I'm having problems doing ghetto tubeless with 4.1d and Mountain Kings.

The rims are so narrow the sealant leaks out before I can get the tyres pumped up. And even using a compressor to inflate the tyre and then a track pump to get it to 60psi the tyres doesn't "pop" and if I deflate the tyre it comes straight away from the rim. The bead is so narrrow on the rim I wonder if it'd be better to trim the 20" tube to 19mm wide?

No sealant in the tires when seating. Use massive amounts of very soap (ie slippery as sh!t) water on the beads. Don't worry, the sealant will mix in with any additional water in the tires. Inflate and seat...you may not hear a loud "POP" on all tires as the tube pads the rim bead hook so visibly inspect the beads for uniform seating. I caution you about going over 40 psi but if they haven't blown of then your probably OK. Leave them seated for a awhile before deflating so the bead can seal up with the split tube.

Then deflate and inject sealant through the valve stem and re-inflate and do the usual ritual to spread the sealant around in the tire.