Hi,
does a Junior member have any chance to become merit sourcer? Of course, if he/she proves the skills for it
It is difficult to determine if you have any scintilla of seriousness in such a question.
Of course, theymos has discretion to determine the appointment of merit sources based on whatever criteria he believes is helpful to him, but he did provide a bit of guideline in the OP, and the most relevant sentence to your situation seems to be the interpretation of "somewhat established member."
Does anything else need to be said?