Try bringing the SRT files into tsMuxeR and Demux them, which will change the SRTs to .SUP files.
If that doesn't work try this software Clown_SRT2XMLSUB
https://forum.slysoft.com/showthread.php?32826-Converting-SRT-%28Text%29-Subtitles-to-Blu-ray
Also need this...http://www.videohelp.com/download/BDSup2Sub511.jar