LessAccounting supports QIF and QFX import. If you have transactions in these formats, you should be fine and just import those files.
In case you have a CSV or Excel file, you can use CSV2QIF or CSV2QFX to convert your files and import them into LessAccounting. You can choose either converter - they work similar and parse your CSV/Excel in similar way.
If you have a QBO or OFX files, you can try to import them into LessAccounting as these are extended OFX files. If LessAccounting does not import them, use Bank2OFX or Bank2QIF to convert. There are also QBO2QIF or OFX2QIF converters you can use. All these converters would be suitable to create importable files to LessAccounting.
If LessAccounting is not importing your QIF/QFX file (not created by a ProperSoft converter), it could be that whoever created the file for you, may not follow the file specifications, so the file is called QIF or QFX, but it is not such inside. In this case, you can try Bank2QIF which tolerates many mistakes in files and create a correct QIF file from your file.