It is possible to import subtitles created in a different software into FCPx.
Now there are two things to it :
(a) Captions
(b) Text Subtitles.
Captions are often required by law and are useful for watching video when you can't hear the audio or when there are accessibility issues (a hearing impaired persons for example).
FCPx can easily import SRT files as File -> Import -> Captions.
But Caption files cannot be formatted. They appear as white text on a black background touching the bottom of your video screen.
ResolverTools here can format your Premiere exported SRT or XML file into Captions compatible SRT file. It can also convert SRT to FCPXML 1.5.
Text Subtitles which appear as text files can be formatted with far more control.
XTitle Importer is another tool that can help you import subtitles into FCPx.
As I use them on a project I will update this post to show what works and what doesn't.