It depends on how the ebuild is built. If it references the version by the ebuild file name, which is very common, you can create an overlay for the ebuild, copy the ebuild to it, rename the ebuild file to have the new version number as part of it, digest the ebuild, make sure the overlay is listed in your make.conf file, then try to emerge it. I did this with Webmin. Yes, I know it's masked and new versions have effectively been dropped from portage; but I use it, and it worked fine.
Maybe I should break it down a little:
Create the appropriate directory in an overlay dir. For Webmin, I had to create app-admin, then webmin:
# mkdir -p /usr/local/portage/overlay/app-admin/webmin
mark@allanon /usr/local/portage/overlay/app-admin/webmin $ pwd
/usr/local/portage/overlay/app-admin/webmin
I then copied webmin-1.510.ebuild from the official portage tree, /usr/portage/app-admin/webmin, to it's new location and filename:
# cp /usr/portage/app-admin/webmin/webmin-1.510.ebuild /usr/local/portage/overlay/app-admin/webmin/webmin-1.530.ebuild
Then digest the ebuild to generate a manifest, otherwise portage will complain when you try to emerge it:
# cd /usr/local/portage/overlay/app-admin/webmin
# ebuild webmin-1.530.ebuild digest
Now add the overlay to your make.conf:
PORTDIR_OVERLAY="/usr/local/portage/overlay"
If you're using EIX to sync/search portage, you'll need to run eix-update after doing this. Now try to emerge the newest Webmin, but first you have to unmask it. I like to use autounmask for that:
Create your package.???* directories (or files) in /etc/portage (I like the dir option), if you don't already have them:
# for a in keywords unmask use; do mkdir -p /etc/portage/package.${a}; done
If you just want the files:
# for a in keywords unmask use; do touch /etc/portage/package.${a}; done
Emerge autounmask if you don't have it:
# emerge app-portage/autounmask
Then unmask Webmin 1.530:
# autounmask =app-admin/webmin-1.530
Now we can emerge it!
# emerge -av =app-admin/webmin-1.530
I've done this on 3 servers to get Webmin on them, and have used this setup every time.
Chances are the ebuild you want to use may be this simple.
Ok, so I scrolled down and saw your reply mentioning the program, jffnms. What I did with Webmin can all most be done exactly the same with jffnms, except you need to modify a line of the ebuild to point to *.tgz instead of *.tar.gz. When I did this, I was able to successfully fetch the gzipped tar file from Sourceforge.
You can use sed to correct it:
sed -i -e 's/.tar.gz/.tgz/g' /usr/local/portage/overlay/net-analyzer/jffnms/jffnms-0.8.5.ebuild
Or download the ebuild I've attached, follow the link you references to create the overlay dir/add to make.conf, etc.
That should work. Let me know how it goes.