Modeling is subject to what kind of slip-joint you are going to use, that is, will control only axial movement or both axial and lateral ?
Some slip joint for abrasive slurry and solid system should not be used for lateral movement. In this case, you can model as CNode with axial free and other five rigid DOF.
Typical Dresser Coupling for general service can absolve axial and lateral(max. 4 deg.) simultaneously. For this case, you can model as expansion bellows or two points CNode with axial and angular free.

Please do not forget controlling pressure thrust and put the guide support for this slip joint system. Also, gasket material is sensitive to solvent and temperature.
If you want to get more information, please contact to any stress engineers in Seoul; SKEC(Jaewon Chang), DIC(Byungkyu Ahn), LGE&C(Seongeun Lee), Samsung(Yongbae Kim).