@J.Sallé
I'm not sure which of the below you mean, so here are both:
⍕¨65 66 67 <-> '65' '66' '67'
80 ⎕DR 65 66 67 <-> 'ABC'
More info:
http://help.dyalog.com/16.0/Content/Language/Primitive%20Functions/Format%20Monadic.htm
http://help.dyalog.com/16.0/Content/Language/System%20Functions/Data%20Representation%20Dyadic.htm