In the case when switching loss is not provided by the datasheet, there are a few options to obtain the data:
1) Build a test bench to measure the switching loss under desired load current and voltage using single-pulse or double-pulse test.
2) Reach out to the manufacturer for the desired data.
3) Estimate the losses using analytical equations based on information in the data sheet.
Several textbooks and application notes provide formulas to estimate the losses as a function of rise time, fall time, reverse recovery charge, etc. Section 2.2 of this application note from Infineon describes a common approach, but it is generally recommended to use the relevant application note from the specific vendor. It is important to be aware of any limitations of the model, for example if it is only valid under certain operating conditions or makes certain assumptions (for example no temperature dependence).
Then, rather than use the lookup table, you can use a formula for the thermal description. Refer to the "Editing Switching Losses" section of the PLECS help. The default function arguments are the blocking voltage v, the device current i and the junction temperature T. You can also use certain mathematical functions (for example abs() to prevent negative loss values).