In short: no.
Unless Zong (or whoever) permits other operators' users to roam on their network, no.
Infrastructure sharing can be anywhere between sharing the physical location (the tower) to sharing the radio/antenna (some countries do this - one radio will broadcast the signal of different operators) or sharing the network (which is essentially roaming or MVNO).
Pakistan, from what I know, only does the tower sharing (the physical location). That's it. And despite this, your signals/network performance may vary depending on
- the strength of the antenna,
- the horizontal angles and vertical height,
- the frequency used by the operator,
- obstructions between your spot and the antenna,
- the congestion at the tower,
- the configuration of the tower (I remember when Jazz switched over from... something else to Ericsson for their 4G, the signal strength changed drastically - some for the worse - but the speeds jumped up drastically as well, even in weak spots. One bar of 4G would give me 20 or 30 Mbps on Jazz whilst on Zong, it wouldn't. This is probably got to do with that QAM and those other fiddly things).
- the network link between the tower and the main servers.
- FBR/PTA/Govt (because they want to feel important as well).