Jordi, the answer on your question depends on what you want to do in the end. Why do you want to use the text keys ?
Since you can use the text keys in your php code only. The system can not processed them if you just paste the text keys into the Page content area. You can find more information here:
http://www.oxwall.org/forum/topic/15100 .
Or may be you are talking about the global variables such the {$site_name} or {$user_name} variables. If you want to add an additional global variable, we suggest to develop the plugin and you can find this post useful as well:
http://www.oxwall.org/forum/topic/11098?page=1#post-56315
If I took you wrong - please provide me with more details.