Update specific position - Char null field !

Hello,

I'd like ask help for this community about a problem to insert a text in a column char field.


The field can be filled with null or with some data, for this reason I can't use substr (in case null or incomplete information, the substr won't return any information to concate and back data to field).

Ex:

Case 1

Field
<null>

Case 2

'a12kok'


Well I need insert information at position 30 ! Both case, I haven't information in position 30, so the final result must be:

Case 1
'*****************************insered'

Case 2

'a12kok***********************insered'

The two examples has 29 characteres, before the text insered ! (imagine * is a blank caracter)

In case 2, we preserved the information 'a12kok'.

Well, I really don't know how to do this, because I did'nt find a function to "insert" a text in a char column !

Could please anyone give me one way ?

Thanks !

Fernando.