I don't see how it would be possible with the current pre-compiled binaries that are available for download; however, the latest and greatest source code has the ability to do what you want without any sort of hacking.
Take a look at the login.go file usage. There are login options that take a user name and password that use SOAP behind the scenes to log in:
force login un pw # log in using SOAP
force login test un pw # log in using SOAP to sandbox org
You can issue the command.
$ force login un pw
Make sure that you include the security token on the end of the password if you are outside of your trusted/whitelisted IP ranges.
I had to compile from source as documented in the Hacking section (copied at the end of this answer for reference), since there are not precompiled binaries. Here is what I had to do on my Windows machine.
The Hacking directions
# set these environment variables in your startup scripts
# download the source and all dependencies
$ go get -u github.com/heroku/force
$ cd $GOPATH/src/github.com/heroku/force
# to compile and test modifications
$ go get .
Following the Hacking directions results in a force executable file (force.exe) being created in the ~/go/bin directory. You can then use that executable.
Once I did that I was able to sucessfully login using the command:
force login firstname.lastname@example.org mypasswordMySecurityToken