What you are explaining is "event-driven" scenario where upon successful completion of something you want to do something else.
Why not use event-driven approach with DirectChannels?
service-activator -> channel
... where 'service-activator' is your service and 'channel' is a DirectChannel.
If your service is not part of the message flow, then you can also use Spring Integration Event adapter where your service (spring bean) would publish ApplicationEvent to Spring Application Context
... and have Event Inbound Channel Adapter initiate the flow:
In the above as soon as you publish event it will be picked up by the inbound adapter which will construct the message and send it to 'input' channel.