If you NEED to have USB-to-serial working NOW and really, really, really can't wait until there's a new official ALARM kernel, then ...
You're going to have to compile your own kernel!
Think carefully! Do you really want to do this?
This is unsupported! Don't blame me if it doesn't work for you! I'm not going to help you fix things!
1) Set up your ALARM installation to boot from SATA. It is way too easy to screw up if you try to flash a new kernel to NAND.
2) Get a serial port connector for the Pogoplug.
If you can't see the stage1, u-boot and kernel messages on your serial port, then you can't find out what is going wrong if you mess things up!
3) Get my 18.104.22.168 kernel patchset from ... http://ppl.ug/BtjZfPDdikc/
4) Read the README!
5) Enable the modules that you want, then compile and install the new kernel (including modules) on your SATA drive.
That's all that I'm going to say about that. Do the research!
If you haven't done enough research to know how to do that, then you shouldn't be playing with this.