[totaltec]

There is a variable called {$types} that you would need to add a new entry to.
Currently it supports:
T => "Text"
C => "Checkbox"
S => "Select box"

You will need to add
A=> "Text Area"

This variable is set in:
admin\contact_us_profiles.php and
admin\user_profiles.php

Then say you added the field to "additional info" well you would need to adjust:
main/register_additional_info.tpl after :
{elseif $v.type eq 'C'}
<input type="checkbox" name="additional_values[{$v.fieldid}]" id="additional_values_{$v.fieldid}" value="Y"{if $v.value eq 'Y'} checked="checked"{/if} />

Add:
{elseif $v.type eq 'A'}
<textarea rows="2" cols="20" name="additional_values[{$v.fieldid}]" id="additional_values_{$v.fieldid}" >
{$v.value}</textarea>

Or something along these lines. There may be additional steps required, I have never attempted this. Hopefully though, this will set you on the right path.