Basically I am asking if we can slot in an existing parachain that has been running for sometime, so has a few hundred blocks into a relay chain like kusama/polkadot. I know the relay chain requires the genesis hash and the wasm runtime as per-requisites before on-boarding a parachain, so in my mind, it doesn't matter if the block number starts from >> 0 ?
For instance, Frontier will initiate database creation beginning with the most recent block number, processing them in a sequence from back to forward until 0.
Similar logic patterns can be observed in GRANDPA or BABE, particularly in the consensus aspect. This will also have an impact on the implementation of light clients.
It is important to mention that many DApps commonly assume that the genesis block is set to 0, which they integrate into their business logic.