Normally a person says kaddish for 11 months after their parent (L”A) is niftar. If, after the whole year has passed by may the person at anytime say kaddish for their parent (not just once, but for an extended period even…)
After the year has passed (or 11 months), there is no custom of saying Kaddish for a parent.
Because there is no custom of doing so, it is not appropriate to say Kaddish for an extended period for a parent; in particular for such matters (of davening etc.) we coshould follow the accepted custom and not invent personal customs that have no source.
However, if there is nobody else saying Kaddish in shul, it is correct to say Kaddish after Aleinu – for “for one’s parent,” but in order that the Kaddish should be said in shul.