The 6DOF model will compute external loads such as aerodynamic and gravitational forces and/or moments, but will not model contact forces. However, as you have said, you could model this for a rigid body by using a UDF for the contact.
You will need the following Macros:
SDOF_Get_Motion
SDOF_Overwrite_Motion
Please look at example 2.6.7.3 in Fluent's Customization manual 2.6.7 Define Contact (ansys.com)
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