Can he enter ihraam for ‘umrah only then form the intention to do Hajj from Makkah?
Question

Can one perform Umrha only as he is about to cross Miqat, stay in Makkah & then perform Hajj-e-Ifrat.
Praise be to Allah.Yes, it is permissible for a Muslim to enter ihraam from the meeqaat for ‘umrah only, then when he has finished it he may exit ihraam, and stay in Makkah until he enters ihraam for Hajj from where he is in Makkah on the eighth day of Dhu’l-Hijjah. This is called Hajj al-Tamattu’, because he has come on his journey to do ‘Umrah and Hajj, and he exits ihraam in between. 
Tamattu’ means that a person comes to do ‘Umrah in the months of Hajj, which are Shawwaal, Dhu’l-Qa’dah and the first ten days of Dhu’l-Hijjah, then he does Hajj in the same year. 
The pilgrim who does Tamattu’ must slaughter a hadiy (sacrificial animal) in Makkah, and distribute its meat to the poor there. If he cannot afford a hadiy he should fast for three days during Hajj and seven days after he returns to his homeland, because Allaah says (interpretation of the meaning): “Then if you are in safety and whosoever performs the ‘Umrah in the months of Hajj, before (performing) the Hajj, (i.e. Hajj-at-Tamattu‘ and Al-Qirân), he must slaughter a Hady such as he can afford, but if he cannot afford it, he should observe Sawm (fasts) three days during the Hajj and seven days after his return (to his home), making ten days in all” [al-Baqarah 2:196]. 
It is permissible for one to do Hajj Ifraad, by intending to do Hajj at the meeqaat and not intending to do ‘Umrah, then he must remain in ihraam until the eighth day, then complete his Hajj, and he does not have to offer a hadiy. 
It is also permissible for him to do Hajj Qiraan, by intending to do Hajj and ‘Umrah together from the meeqaat, then when he reaches Makkah he should do tawaaf and saa’i, and not exit ihraam, rather he must remain in ihraam until he has completed the actions of Hajj, and he must offer a hadiy like the one who is doing Tamattu’. These are the three types of Hajj, and each of them is permissible, but the best of them is tamattu’. 
This has also been discussed in the answer to question no. 31822 and 27090. 
And Allaah knows best.