Short answer is you can not.

With UA, alot of credit card purchase ($180,000 to $240,000 depending on the card) and 4 UA operated flights
The HSBC Star Alliance Credit Card provides *G/ UA for $4,000 first year spend and then $60,000 annually spend but is for Australian residents only
Many *A carriers offer *G for 50,000 miles traveled still (A3 and TK are a bit easier -- but still require a lot of flying)

WIth UA you can purchase a select *G benefits separately; Premier Access, EconomyPlus, check bag waivers, ... with a combination of UA subscriptions and credit cards. (The Club Card and E+ subscription, total $1,600 provides some of the major benefits on UA operated flights.) But only the lounge access carries over to other *A flights.