How would I add a block or pipe, to allow conversion between my mods power system, and Buildcraft MJ?

Use Buildcrafts API.

No durr genuis.

Oh, THAT interface would be nice to use.

What's your question though.  Do you not know how to install an API and use it?

https://raw.githubusercontent.com/coolAlias/Forge_Tutorials/master/ModdingWithAPIs.java

 

^^ The above guide helped me figure out how to implement BC compatibility in my mod (I did not write it, please give credit to coolAlias )

 

(https://github.com/shadowmage45/AncientWarfare2)

 

 

Edit:

 

Would like to add that this seems like the 'proper' way to do mod-api dependencies.  Using the @Optional annotations makes your code design and packaging so much cleaner and easier to deal with in the long run.  It can mostly do away with all of the proxy classes and reflection that used to be necessary even for simple dependencies.