While I would prefer a way to replay my digital purchases from the Wii/Wii U, a disc drive add on alone will not get it done. Most of the games would have to be retooled due to controller issues in order for them to be compatible.
I do hope they find a way to bridge the divide between those of us that purchased on the Wii/Wii U and also got the Switch so we aren't just rebuying the same software in a loop but there are way too many hurdles blocking it beyond just the software physical format.