For the UV attribute you need to indeed use the Store Named Attribute node currently. I’m not sure why the 2D vector option is not available in the nodegroup outputs to be honest though.
The issues with the conversion between legacy curves and the new curves will be resolved by proxy at some point as the old system is being phased out in favor of the new once the features are all there. Of course, we should have some operators for conversion in the meantime though.
But keep in mind that geometry nodes already has a lot of functionality for this exactly. For your proposal you can for example just use an Attach Hair Curves to Surface modifier after piping in the legacy curves geometry on a new curves object.
Of course, that’s not a builtin operator. But as I said before, the idea is not to have both systems coexist in the long run. So fostering an environment where both are kept compatible at all times is at least to some degree misused development time.