From what table will you be able to use the request for quote number to find the data that you want to enter into the quote line item table?
Are these records already entered in this table, but with an ID number (request for quote number) common to all that you want?
If so, there are two basic ways you can do this. Both use a script and start with this step:
Set Variable [$RFQNumber ; value: Request item::Request for Quote Number]
Both would use a global field: gTemplateRFQ in order for the user to specify the Request For Quote number of the line items that you want to copy from.
In each case, you'd then use gTemplateRFQ to perform a find for those line item records.
The next step would be to use Import Records to import this found set into a different occurrence of the same table. You'd then use Replace Field Contents to update the records created by this import so that Request item::Request for Quote Number gets the value stored in $RFQNumber.
The next step is a looping script that uses duplicate record to duplicate line items one record at a time and uses set field to update the Request item::Request for Quote Number of the new copy with $RFQNumber.
Either way produces the same results and the user gets those results by selecting/entering a request for quote number in gTemplateRFQ and then performing your script via trigger or button click to cause the complete set of records to appear in the Line Items portal.