No, it is not possible to move an ArcGIS StoryMaps story created in ArcGIS Online to Portal for ArcGIS due to the differences in version and functionality. Stories in ArcGIS StoryMaps contain elements such as text, uploaded media files (images, video, audio), legends, express maps, web maps, web scenes, and layers. To successfully move a story, all these elements must have the ability to be transferred, as well as function in the same way. Due to certain features in ArcGIS Online being unavailable in Portal for ArcGIS, moving a story from ArcGIS Online to Portal for ArcGIS is not supported.
The ArcGIS Online version of ArcGIS StoryMaps is ahead in releases and updates in comparison to Portal for ArcGIS. There is, therefore, a gap in terms of the features available between the two platforms. Refer to ArcGIS StoryMaps: ArcGIS StoryMaps on ArcGIS Enterprise feature comparison for more information. If the original story must be retained, Esri recommends leaving it in ArcGIS Online so all the features remain accessible. However, if moving the story from ArcGIS Online to Portal for ArcGIS is necessary, the story must be rebuilt, and limitations in features are to be expected.