Best way to make the port install another port

[Big Lebowski]

Hi,

I wanted to add an option to one of my ports to optionally install
commandline tool, that's in another port. Doing the options part is easy
and clear, but how should I add that port to build procedure to do it the
right-way? The most obvious way would be to add RUN_DEPENDS+=, but the
original port doesnt really depend on this tools...

Another question is to where is the best place to put such dependency? Post
install, pre install?

Thanks in advance!