You can indeed build plans manually in Intervals.icu with the workout library, as Gerald described.
If you are interested in something more automated: I’m developing an external tool that can generate complete training plans and sync them directly with Intervals.icu via the new OAuth API.
I posted a short description in the External projects section here on the forum:
External Projects → DunMove – automatic training plan sync with Intervals.icu
No pressure of course – just sharing in case it might be useful.