Do you mean create a separate server program optimised for dialup? To reduce lag is one of the team's aims with each successive version, so I gather, so this would be somewhat pointless.
Or do you mean that server admins should offer two servers - one for dialup and one for broadband? If so, this would cost a lot of money and many admins wouldn't see the need. Plus the quality and availability of broadband is increasing all the time.
insnt there a gta3 multiplayer mod available already?
Yep, there's GTA3:MTA 0.3b.