oh that's a complex topic. you need a SMPTE-ready XML file. but as far as i know openDCP as well as other apps only offer 100% support for interOP XML files. you need to either write that XML yourself and hope for the best of buy "belle nuit subtitler" for example. anyways: subtitles in SMPTE are still buggy everywhere. interOP means 2K, 24p, 5.1 in a flat container only!
there is also this online tool (didn't work for me though), you might need another software to rewrap the DCP with subtitles or do everything manually (be careful!):
http://www.michaelcinquin.com/tools/DCPp.s. there are lots of topics about subtitles here in the forum! feel free to roam!