These inputs would take POLYURETHANE_GM27 and assign the properties of nitrogen and then mix it with the DIESEL.
I suggest you take this slowly. First, get "DIESEL" to work with the correct properties (e.g., heat of combustion, etc.) and make sure you get the correct HRR.
Next, do the same for your POLYURETHANE surrogate. Note you will have to specify thermodynamic properties, as this is not pre-defined FUEL in FDS. Again, check that you get the correct HRR for a case with a simple REAC line.
Then, for your option (1), you basically have things right, just mix the SPEC into a new SPEC (but you should use LUMPED_COMPONENT_ONLY=T when defining the DIESEL and POLYURETHANE, search the user guide for LUMPED_COMPONENT_ONLY).
For your option (2), you will need to define your own REAC lines with stoichiometry. This is discussed in the user guide. Follow the molar stoichiometric coefficients for the REACs of your individual runs.