The first thing you need to know if you want to crossfade is that there is function for this in the ‘Video Sequence Editor’.
The remaining problems are: how can I transform my scene in a clip? And: How do I select the part which should be affected by this effect.
And now let us begin:
First we need to switch to the ‘Video Sequence Editor’. Now add the scene as a clip. For this click on ‘add’ and select the scene.
You will get one strip but this is not enough for the crossfade effect so you have to split the scene into the two parts you want to crossfade.
This can be done with moving the green line to the right position and searching under the ‘strip’ option block the right split option (hard or soft).
Afterwards move the right part one row upwards (higher numbers overlay lower numbers) and overlap the area in which you want to have the effect.
The last thing you have to do is to add an effect strip (for the cross effect you will need to select both strips).
Such an effect strip can be found under ‘add’ ->effect strip and in this case: ‘cross’.