Use a dropping funnel.
This is how we used to handle it:
1) Insert inert gas line into t-butyl lithium
2) Insert a cannula with one end into a dropping funnel (with a septum put on it), and then the other end into the bottle
3) Fill the dropping funnel to the required volume and then remove the cannula.
4) Then you can dropwise add the t-butyl lithium from the dropping funnel which is also under inert atmosphere.