Hi there Coleen,
Maddie here from monday.com support.
Unfortunately, this is not supported on the platform at this time.
In order to convert multiple items to a Drop-Down column, it would require you to use our API.
If you are interesting in learning how to develop this for your account, here are some resources to get you started with our API and developer framework.
Head over to our API documentation for a detailed look at our API, including a list of the operations to get and update data from your monday.com account.
Webinar: monday.com for builders
You can also check out our recent API webinar here, which goes through some API use cases and basic API calls.
If you need additional support on our APIs or apps platform, I would recommend doing a search for your question in our developer community here. You can also post to the community to get help from other monday.com developers. The community is managed by members of our dev and CS team.