I think your missing the point here. You can adopt a child anywhere. But will you be able to bring him to the USA is the real question.
Steps
1.You need to abide by the laws of Nepal to adopt a child. (Go through Nepals child services for approved agencies).
2. Now you want to bring your adopted child to USA . Will the US recognize the child u adopted in Nepal fit to be processed for immigrating to US.
3. Let's say the child is fit to be adopted. Does your status affect his immigration processing? If you're a CITIZEN OR GC

The ministry of Nepal / child services will allow you to adopt through its agencies. The government of Nepal doesn't allow street children's to be adopted overseas. But it will allow children's abandoned in hospitals to be adopted overseas.
But the U.S. won't allow children's abandoned in hospitals to be bought to USA, but it will allow street children's to be adopted. (The link by unicorn states that).

Now your best bet is to,adopt children's whose family are dead (but through agencies approve by Nepal govt). Let's say u adopted such child .

since you're a GREEN CARD , THE CHILDE SHOULD HAVE BEEN adopted by u before his 16th b'day and he/she lived with u for 2 years.
But if you're a citizen/wait till u become one then you dint need to worry about the 16th b'day rule.
Also u can adopt a child whose parents are dead (without the objection of NEPAL & US GOVERNMET)



Sorry it's,unorganized ..