Windjam

"rank pari passu with" means "have the same seniority as"

so this new debt will rank at the same level as unsecured debts but below senior debts.

How does the old lender agree?
normally, the lender of the senior debt will have a clause that the company will have to get specific permission to have new debts ranked pari passu plus the lender will typically have secured all assets of the company legally, so if there is no more assets to secure.

In practice, the company will go to the lender (bank) and ask permission. A negotiation will ensue and the company have to give up sth to get this permission.