Local libraries/packages with dub: How?

[z]

How does one set up a dub package so that it contains multiple 
sublibraries that may or may not depend on these same 
libraries?(but never co-dependant)
So far i've tried using add-local and add-path with subpackage 
declarations in the root folder's dub.sdl but to no avail.
(dub does not complain but silently doesn't add dependencies with 
-v, dmd just errors out with the standard file not found error 
"module y is in file y.d which cannot be read", with the path of 
my libraries absent from import paths.)

Alternatively, is there an equivalent of -version= for DUB's 
build command?(for when a package author didn't add a config that 
incorporates the desired versions.)

Big thanks and my apologies if this is the wrong place for DUB 
discussion.