It’s currently not possible, and Discourse doesn’t support colspan or rowspan attributes for <td> and <th> elements. I don’t think it’s the roadmap either.
It is possible that you can add the support for this with a custom plugin. See: Whitelisting HTML tags / attributes
the list of supported HTML elements is here: https://github.com/discourse/discourse/blob/main/app/assets/javascripts/pretty-text/addon/allow-lister.js#L115. The table tags seem to be here: https://github.com/discourse/discourse/blob/main/app/assets/javascripts/pretty-text/engines/discourse-markdown/table.js
Maybe this will be of some help.
If you want to request allowing colspan or rowspan attributes in core, you may want to create a new Feature topic. 