You can import PNG graphics to LibreOffice Draw and save it on OpenDocument Drawing format. It might also be possible to use one of the on-line conversion services and perform the same conversion there.
Convert Portable Network Graphic to OpenDocument drawing.
We found 2 software records in the database related to .png to .odg file format conversion.
Multiplatform software - convert png to odg on any platform
A vector graphics editor
Convert from png file format
The tables with software actions are a good pointer for what a certain program does with various file types and often may give users a good hint how to perform certain file conversion, for example the above-mentioned png to odg. However, it is not perfect and sometimes can show results which are not really usable because of how certain programs work with files and the possible conversion is thus actually not possible at all.
Graphic and diagram software from LibreOffice suite