One is to use the Higgs to give neutrinos mass. For technical reasons this only works if there are both right and left handed neutrinos. We have only ever detected left handed neutrinos, so you'd have to also add right handed neutrinos, and just say that they don't really interact with anything else.

The second way you can do it is add a very heavy Majorana particle to your theory for each of the 3 neutrinos we know about. These Majorana particles are their own anti-particle (just like the photon is) and as a result are able to have a non-zero mass without the Higg's mechanism. The three types of neutrinos we already know about would then get their masses as a result of some slightly complicated maths involving the masses of the three new Majorana neutrinos.