It is not easily possible to install php v5.3 into an old appliance that does not support it OOTB (it is possible, but not advised - lot's of work, lots of risk of breaking stuff and lots of maintenance headaches for the future...)

I suggest that you upgrade to the newest appliance - v12.0. It has php v5.3. In fact the Joomla appliance has v2.5 of Joomla.

You could try migrating using TKLBAM, but you may need to tweak some stuff to get it to work. There are proabably other options for data migration too (but I don't use Joomla so I have no idea...)

One way of upgrading could be installing the new appliance and then copying your Joomla folder from the old appliance to the new version. After that you export your joomla database with phpmyadmin on your old appliance and import it on the new one. This works only if both Joomla versions are the same.

Doing a Joomla update (i.e. 1.6 to 1.7 or 2.5) this way is not recommended because the table structure may change.

Don't forget to reconfigure apache too (or backup + restore configuration).

Which Joomla version are you using?

If you want to upgrade Joomla, check the Joomla documentation:

http://docs.joomla.org/Migrating_from_Joomla_1.5_to_Joomla_2.5

http://docs.joomla.org/Upgrading_from_Joomla_1.7_to_Joomla_2.5