The $srcObj array is initially populated using attributes from the source object referenced in that specific step. That being said, it is simply a PowerShell construct.
If the $srcObj array is updated with a new attribute (one which does not already exist on the source object), then it is possible to access this attribute value in a different Sync Rule.
This will not modify the source object itself, only the array which has been constructed to represent that object.
In the first Sync Rule, use the following script:
$srcObj["tempVal"] = 'SET'
The tempVal attribute can be anything which is made up for this purpose. The attribute can be named anything, as long as that name doesn't already exist as an attribute in the source.
In a following Sync Rule, access this value using the following script:
This will return a string with the word SET