???Question???
For example, if I ask for a 6BTC Loan, can I offer to pay it back over the course of 12 months? (including interest, of course

)
Of course they're allowed. The problem you'll find is that you're unlikely to get a loan (certainly not a 6
BTC loan) unless you've got collateral worth at least 120% of the loan amount.
Also, paying in installments leads to higher fees, which in turn leads to higher interest rates for you.