I am not sure I understand your question.
Currently, you can use the "Explicit Retention Time" and "Explicit Retention Time Window" values to tell Skyline where to expect your molecule to elute.
It sounds like you might be asking about specifying a "Left Explicit Retention Time Window" and "Right Explicit Retention Time Window" so that the explicit retention time window would not be centered on the Explicit Retention Time value.
There is no way to do this in Skyline. We always imagined that you could change the Explicit Retention Time value so that it was in the middle of whatever window you wanted to achieve.
Is something going wrong when you do that?
Can you send us your Skyline document?
In Skyline you can use the menu item:
File > Share
to create a .zip file containing your Skyline document and supporting files including extracted chromatograms.
If that .zip file is less than 50MB you can attach it to this support request. You can upload larger files here:
https://skyline.ms/files.url
It might also help if you could send us a screenshot of what you are looking at.
-- Nick