Libtorrent has a specific setting for this, however, I wasn't able to find any info regarding Transmission.
The reason for outgoing connections is to enable seeding from behind a firewall/NAT without port forwarding.// determines if seeding (and finished) torrents should attempt to make
// outgoing connections or not. By default this is true. It may be set to
// false in very specific applications where the cost of making outgoing
// connections is high, and there are no or small benefits of doing so.
// For instance, if no nodes are behind a firewall or a NAT, seeds don't
// need to make outgoing connections.
bool seeding_outgoing_connections;
This is especially important for private torrents (with DHT/PEX disabled due to tracker requirements), since DHT connections cannot be used to seed such torrents.