Hello, thank you for getting in touch and for your support!
It's definitely possible but it will require you to manually perform the lighting calculations yourself, which might be hard to tackle since you have less lighting nodes to work with, as some of those that we provide are not compatible with the SRP.
The Color port in the main output node may be used to receive the final output of your calculations for this as well.
Regarding the warning, some of our lighting nodes are only compatible with the Custom Lighting mode for the Standard shader, you can find more information in the light nodes
wiki pages. This also means that they can't be used in other contexts, such as other templates.
We do have plans to add SRP specific nodes that will assist users in creating custom lighting as in the Custom Lighting mode, but I'm afraid that we don't have any ETA regarding that at this time, apologies for the inconvenience.