You can add a boolean field to your source table and display this field on the list.
A second way could be using MARK for marking the selected records:
1) Create a page field called "Selected" with SourceExpression = MARK.
2) Add an action call "Select" on the page
3) In the OnAction trigger add a local variable that points to your table (here called YourTable) and add the following code:
CurrPage.SETSELECTIONFILTER(YourTable);
IF YourTable.FINDSET THEN
REPEAT
GET(YourTable.XXX); //Here you've to select the record by primary key, I've called the primary key XXX
MARK(TRUE);
UNTIL YourTable.NEXT=0;
To deselect the record, add a new action with MARK(false).
