You can add it to the cmp/standard.mos file in the LTC lib directory and it will show as a part of the NMOS list when you put the NMOS device in your schematic.
Here's what I put in the standard.mos file and it seemed to work normally as an NMOS device:
.MODEL IXTT20N50D NMOS
+ LEVEL=3
+ L=2.0000E-6
+ W=5.5000
+ KP=1.0446E-6
+ RS=1.0000E-3
+ RD=.22202
+ VTO=-.89028
+ RDS=20.000E6
+ TOX=2.0000E-6
+ CGSO=3.5684E-9
+ CGDO=37.622E-12
+ CBD=4.8729E-9
+ MJ=1.5000
+ PB=2.6055
+ RG=10.000E-3
+ IS=1.3714E-6
+ N=2.0283
+ RB=1.0000E-9
+ GAMMA=0
+ KAPPA=0
+mfg=IXYS-Depletion
+Vds=500V Ron=330m Qg=78.5n)
You should make a back-up copy of the standard.mos file before you do this.