@Pieter Kruithof - This is not currently possible using native Miro functionality.
One workaround is to group your imports in your spreadsheet, import them, select them all quickly using Shift + drag, and apply a tag, and repeat.
Miro Developer Platform
This should be possible using the REST API, and maybe there is an option to execute this from a spreadsheet in the board by combining the SDK + API? Here’s a link to the Miro Developer Platform documentation → https://developers.miro.com/docs
Zapier
Maybe there is a way to do this with the Zapier + Google Sheets + Miro integration:
https://zapier.com/apps/google-sheets/integrations/miro
Wish List Post
I am not initially seeing a Wish List Idea posted to request this - however, my search skills may be weak this evening.
I’d recommend adding this as an Idea in the Wish List category by following the guidelines here: Wish List: Everything You Need to Know.
If you do create a Wish List Idea post, I would also recommend that you post a link to it back here, so that future readers of this post can quickly get to and vote for your Idea.
Lastly, don't forget to vote for your own Idea.