Do you mean File->Manage->External Data Sources->New->Add File ???
And you can add references to tables in the external file in Manage | database | relationships by clicking the far left button on the bottom of this window and selecting "Add filemaker data source" from the Data Source drop down. Now you can link this table in relationships to other tables just as though the table from the external file were defined as part of your current file and you can set up layouts to it, modify data in scripts and so forth.
If you just want to open the other file and select a specific layout in that file, you can do that as well.
Create a script in the external file with this script step:
Go To layout [Get ( ScriptParameter ) ] //and use the "Layout name by calculation" option
Then, in the first file, use Perform script, select the option to perform a script in the other file and specify this Go to layout script. Pass the layout name as a script parameter. (Or if you always want to select the same layout every time with this script, you can just specify the layout in the Go To Layout step and not use the script parameter.)