Using spaces to indent is the default outline format, possibly because not all basic text editors recognise tabs.
There are a few options depending on whether you can live with Word-style indents or if you really want tabs. If the former is OK you could just send the branch to Word (or if you have WordX set up that branch only for export). You can either set up indented styles in the Word template used for export or in the case of the built-in export you can also set the level down to which indenting will be applied in the exported document.
If you really want tabs however there are two work-arounds. When a branch is pasted into Word, MM consistently indents the text by two spaces for each level. Therefore if you know for certain that there are no double spaces already in the topic names you can simply copy the branch, paste it in a Word document and search for and replace all double spaces with the tab symbol ^t.
If however you suspect there may be double spaces in the topic titles you will have to go a slightly more round-about route via Excel. Send (rather than copy) the branch to a new CSV file using the Outline format. Open the CSV file in Excel, copy the table (without the column headings) and paste it in Word. Click inside the table in Word and from the Table Tools Layout Tab click on Convert to Text and select "Separate text with Tabs".