I know this is an old thread but it is a question I've seen asked many times over the years and I have only just got round to adding the unit conversion function. I've been using this method for quite a while but, as I was born in the UK after 1970, I have never used feet & inches.
Firstly, in AutoCAD LT, you will need to temporarily change your unit display format to decimal [LUNITS=2]. Getting Excel to isolate a string such as "2'-6 3/4"" and convert it to the value of 30.75 is a challenge for another day or, more likely, another person. Feel free to have a go.
Use the LIST command to display the properties of all the lines, polylines, lwpolylines, circles, arcs, ellipses and/or elliptical arcs, the lengths of which you wish to add together. Press return when prompted until the entire list is displayed on the text screen and then copy the text to the clipboard.
Note: It is easier to select these objects if you are able to isolate them on their own layer. Any other types of object you select will be ignored when calculating the total length but there will be more text to copy to the clipboard.
In the spreadsheet (See attachment) set the AutoCAD units to match whatever you used in the drawing. If you used the Architectural or Engineering unit display format then this will be inches.
Set the units in which you wish to have the total length displayed and the number of decimal places to show. The spreadsheet will do the conversion for you.
Finally, click on the button labelled 'paste & calculate'. The object properties text will be pasted into column A and the formula in column B will analyse the text & extract the relevant length data. The total length will then be displayed in the green result box in your chosen format.
I hope I've explained it clearly enough, that you all find it useful and it saves you lots of time.
The conversion table is on Sheet2 should you need to add more unit types. Inserting rows or columns in the table rather than adding data to the ends will ensure that the unit descriptions are automatically added to the drop down lists on Sheet1.