How to store a sequence of events related to a transaction
Why not just have a transactions table:

CREATE TABLE dbo.Transactions(TransactionID INT IDENTITY(1,1));


When you're about to start a new transaction (a set of imports and exports), do this:

DECLARE @tID INT;
INSERT dbo.Transactions DEFAULT VALUES;
SELECT @tID = SCOPE_IDENTITY();


Now in your import and export tables, add a column that holds the transaction id, and store @tID there for every action in this sequence.

Or use a separate table to store Transaction + Event combinations, if events can be recycled. For example:

CREATE TABLE dbo.TransactionEvents
(
  TransactionID INT,   -- foreign key
  EventTypeID TINYINT, -- 1 = import, 2 = export
  EventID INT,         -- implicit foreign key to import/export table
  ... other columns ...
);


You can have a lookup table for EventTypes if you want, e.g.:

CREATE TABLE dbo.EventTypes
(
  EventTypeID TINYINT PRIMARY KEY,
  Name SYSNAME UNIQUE
);
INSERT dbo.EventTypes VALUES(1,N'Import'),(2,N'Export');


You might also consider not having separate import and export tables, but one events table with a type of import or export. Then the foreign key here can be explicit.

And please ditch the tbl prefixes. All they do is make you type three extra characters every single time.