Is it possible? I have downloaded the expr dll file but I don't know how to get it to work with libpd.
Very new to libpd and any other kind of coding apart from PD.
I am using Processing which is more or less like Java.

The expr family has been part of vanilla PD for a long time, so I would assume they are included in libpd?

If not have a look at this: https://github.com/libpd/libpd/wiki/Adding-Pure-Data-external-libraries-to-your-project

Hmm maybe I need to give up on Processing/puredatap5 and move over to Java or something else. It seems pretty much unsupported and not updated for a long time.

What is not supported or updated? libpd is actively developed and seems to have parity with pd-vanilla 0.51 which most certainly includes [expr] objects already.

You shouln't need to download a dll to make this work. Have you tried by simply using an [expr] object in your patch running on libpd?