Some time ago I wrote a one liner to import OPML files. Now it has become a lot easier to import an OPML file with ek.
ek import opml subscriptions.opml
It is also possible to export an OPML file.
ek export opml > subscriptions.opml
With these two commands it becomes easier to import and export OPML files.
The format of the OPML files should match the structure of the Microsub channels and feeds. Microsub has a list of channels and each channel contains a list of feeds. If the OPML file contains feeds on the first level of the file, it will skip these. Channels in the second (or higher) level will also be skipped.