Designing & building a pressure hull is probably not beyond the capabilities of Pakistan. But what about the other crucial items such as:
1. Propulsion system
2. Command & control systems
3. Weapon systems
4. Noise control.
Sophisticated military hardware; be it a tank, aircraft or naval vessel is not everyone’s cup of tea. Even a simple item such as an assault rifle requires millions of dollars in research, hence most countries manufacture assault rifles under licence.
Pakistan’s industry is at an embryonic stage compared to India’s. LCA project began in 1980’s but had to wait 35 years before Tejas could become operational and even then Tejas relies on imported US power plant. Arjun tank was conceived in 1974 and is finally entering service now but it costs double the comparable Russian alternative. Even a country like Japan failed to improve on F-16 and its F-2 fighter roughly costs 4 times the comparable F-16 Block 52.
Development of submarines is no different. Indian Navy expansion program (project 75) consists of building 6 Scorpene submarines designed & engineered by France.
I don’t mean to find fault with the Indian or Japanese military technology; my point is that given sufficient time & resources; Pakistan’s indigenous submarine could become a reality, but it won’t be cost effective and a waste of already scarce resources. A far simpler & cheaper solution is buying it from China or whosoever is willing to sell submarines to us.