I'll see if I can make this the short version.
First check if the dependencies in the PKGBBUILD has arm versions at arclinuxarm.org under Packages.
If they do, then simply add armv6h armv7h and aarch64 to the PKGBUILD's arch=() line.
If some of them don't, you need to "port" over those aswell, by adding the architectures to their PKGBUILD aswell, build and install them on your arm device and then build the package in question on your arm device, like mentioned above.
BUT, some software can not be built on ARM in general, because of how it is coded, so we are fresh out of luck on those.
Most software can, and we are not afraid to add packages to the build server for our own community repo, if it's requested.
PS: Quickest way to get a package into our repo is by supplying us a working PKGBUILD.