Fellas, I did a thesis on the UN 1 of whose chapters covered this exact topic, so I'm pretty familiar with this issue. There are plenty of int'l law journals about the UN having its own standing army, including with the implementation of Art. 43 agreements (no country has ever signed 1, so it's all a dead letter) which allow the UN to have national armed forces signed over to fulfil the Charter's Ch. VII peace-enforcement purposes directly under its command and control, and other proposals under Art. 97 iirc for the UN to have its own directly-recruited military force, similar to the Secretariat, although such a force would probably only be speculated at approx 5,000-10,000. Such latter proposals, which included a 1993 article written by former Under-SG Sir Brian Urquhart (who had also been a 1st Airborne Div intel officer leading up to Op MARKET GARDEN) discussed the possibilities of hiring Gurkhas to fight for the UN, in the same manner as they serve the British Army, or to directly recruit individuals in the same manner as the FFL. Thus far, I believe the only time the UN has recruited its own armed personnel directly was with the lightly-armed 500-strong UN Guards Contingent in northern Iraq from 1991, who took over from the coalition Op PROVIDE COMFORT force providing humanitarian relief to Kurdish refugees.
Hmmm, maybe 1 viable way you can begin to have a UN standing army a la foreign legion, but not of course of the sizes you're mentioning, would be to have the UN from the early 1990s subcontracting private military cos. like EO and Sandline after seeing their phenomenal success in combatting war crimes-committing irregulars in Sierra Leone and Angola, and perhaps even if the SC had adopted ADL's 1994 proposal to employ private security guards to disarm the Hutu armed extremist elements in the Zairean refugee camps. There are BTW also many int'l legal journal articles discussing this concept.