The Arrhenius rates are already a function of temperature. You just need to give the related coefficients, like A and Ea. The documentation of MFiX (4.14.4. Implementation of CHEMKIN mechanism in MFiX — MFiX 25.2 documentation) gives all the details about how the reaction rates are calculated in MFiX based on the coefficients from the users.
If you want the coefficients, like A or Ea, to be a function of temperature, you do need to use UDF instead of the built-in CHEMKIN ability in MFiX. MFiX has some tutorials for reacting flows. After opening MFiX GUI, go to “New project” and choose the reacting cases, like “2DTFM silane_pyrolysis” or " 3D PIC silane pyrolysis". Those tutorials have the UDFs, usr_rates.f or usr_rates_des.f for the reactions.