how do i convert GeoJson to IMDF format

It's not easy to give a complete answer to your question since you're not telling us anything about the source data and workflows like these can be notoriously complex.

IMDF has a fixed schema, so the feature types are pre-determined. What you'll need to do is identify whether or not the data in your GeoJSON fits those feature types and then more or less connect the dots. It seems there's an IMDFValidator custom transformer available on the FME Hub which I think would be a good idea to use (the one time I had to work with indoor mapping data my source CAD files were so poorly made I pretty much gave up on it)

I highly recommend you start trying it. If you run into issues and have more specific questions it'll be easier for us to help you. In that case it would also be good to post some sample data and describe what you have done so far.

Welcome to the FME Community @rshah! I agree with @redgeographics, can you tell us what you've tried so far? Let us know what errors you are getting or if you are getting an unexpected output.

Have you checked out the Creating and Validating IMDF Datasets tutorial series? This might help.

Have you had any luck reading your GeoJSON dataset into FME? This would be a good starting point.