I don't think you need to. SAIL is doing what they ask I think. The two "T"s are independent. Let me exemplify. Here are the three dials sail will do (these aren't syntactically correct but you get the idea).
internal call
dial 401,Tt
This allows either end to transfer the call using DTMF
outbound (from internal to external)
dial somenumber,T
This allows the caller ONLY to transfer the call using DTMF. This is correct when you think about it since you don't want the external callee to be able to initiate a transfer
inbound (from external to extension)
dial 401,t
This allows the callee (extension) ONLY to transfer the call using DTMF. This is correct when you think about it since you don't want the external caller to be able to initiate a transfer.
In any event, it is my understanding that FOP initiates transfers using the AMI so I think this is all irrelevant, at least as far as FOP is concerned, but as I said I'm not an expert.
SAIL can/does generate the control files for FOP when it creates sip.conf and we have quite a few customers who use FOP2. However, having said that, I can't think of any who use it to do drag/drop transfers and this is why I can't give you an absolutely definitive answer. Have you spun it up and tried it yourself or are you still in the planning stage?