Hello,
A somewhat quick and dirty way::
In[113]:=
ToStringPad[n_, len_, pad_] :=
FromCharacterCode[
PadLeft[ToCharacterCode[ToString[n]], len,
First[ToCharacterCode[pad]]]];
ToStringPad[1, 3, "0"]
ToStringPad[22, 8, "="]
Out[114]= "001"
Out[115]= "======22"