Well theoricaly, it is possible to grow the device.
mdadm which is the command used has the possibility to do it (-G option).
However, you need to do it by hand after getting a shell on the machine.
Not for the faint of heart.
Then, once the /dev/mdx is grown, one need to use the resize2fs to grow the filesystem.
This command is not provided in the official firmware. Have a look at the fun_plug distrib to get it.