I'm not familiar with SQL Squirrel but it's not clear why you can't read the "SH." prefixed files.
One possibility is the "." operator is the default separator between library (schema) and file (table) names when using SQL naming.
Try using quotes around the name, for example:
SELECT * FROM QS36F."SH.ITEM"
Also if you need access to specific members (partitions) you will need to create an alias to query against:
CREATE ALIAS QTEMP.M131204 FOR QS36F."SH.ITEM" (M131204);
SELECT * FROM QTEMP.M131204;
Due to the use of the QS36F library it's also possible these are not 'externally described files'. That means the tables may have only one big field to hold all of the individual field data and it is broken apart in the programs themselves.
DBVisualizer is a great third party SQL tool that can access a multitude of databases.