Display lines from multi line text field as multi choice field

Since that's very difficult or impossible, natively, , can I suggest that you tell us the workflow or business problem you are trying to solve? Maybe there is an easier different viable approach.

HI Mark, nice to hear from you again.  We deal with many clients, and any client can at any time ask us to take care of multiple requests.  Each request is identifiable by what we will call a "serial "number", but there are potentially thousands of "serial numbers", and keeping a central database of all of them would be a waste.  We manually input several numbers at a time into a multi line text field, but then, almost immediately, the project will split, and the developments for each serial number must be tracked individually.
For when things don't have to be done on the fly, I've arranged a child table in which there is a lookup field for all the serial numbers, and a formula text field which by default contains all of the serial numbers, but is replaced by only several when those are copy/pasted into a "work on these serial numbers" text box.  The purpose of this arrangement is that 50% of the time, all serial numbers do follow the same flow, so the default setting eliminates the need to copy/paste in these cases.
My main issue is when things have to be input quicker, as some situations demand, where the "copy, paste, and start a new child record for the other serial numbers(s)" procedure is a but too bulky.  That is why a multi choice, or a multi select, would be ideal. 

I have an idea, I'll post back to let you know how it's working.  When the serial numbers are first entered, they should be separated by a comma.  There will be a formula field for each possible serial number (there shouldn't be more than 12 at most) which pulls a specific serial number using the 
"part" function.  Those field will be used as lookup fields on a child table, where I will use the "copy this field" function for every field, twelve times, and label and arrange them on the form so that they correspond to a specific serial number.

The above solution seems to work.  I have a text warning on the form that all serial numbers must be separated by a comma, and the total number of serial numbers may not exceed thirteen.  Next, I have thirteen formula text fields, each one identifying the "part" before a comma.  In addition, i have thirteen check-boxes, and each one is named to correspond to a single serial number.  That way a user can identify the serial number he/she would like to act upon.