Ok, first, all the "default" planes in FSXA are in FSX:SE already. No need to import anything.
Now, if you mean you can't import ADD-ON planes you had in FSX, that's an entirely different story. If they are payware, you have to re-install them into FSX:SE. As though you just got them from their manufacturer.
If they're freeware, just copy them over to the FSX:SE Aircraft folder, from the old FSXA Aircraft folder. Make sure you copy over the gauges in the Gauges folder, and Effects folder over too. You'll want to install any Modules you had in FSXA too. Things like FSUIPC, and so on.
Or re-install them, your choice. Freeware planes will install to FSX:SE just like they did to FSXA, as long a you point the installers to the right folder. Remember that Steam kind of buries FSX deeper in it's folder structure than normal FSX is.
IF, and this is a BIG if, you allowed Steam to install to C:\Program Files or C:\Program Files(X86), you've got other problems. More than likely a permissions problem. Windoze, especially Win10, has some real personal problems allowing any kind of changes to programs in those two folders. That includes adding things to them.
If they're in one of those two folders, it would be best, IMO, to uninstall completely. Not just FSX:SE, but the whole folder structure Steam installed. Then allow it to re-install to another folder. ANY other folder than those two. C:\, D:\Games, whatever.
Finally, if you just can't find the normal, default, planes you're looking for in FSX:SE, the Select Aircraft menu page, make sure you've got the "Show All Variations" radio button checked, lower right area of the page.
Let us know how things go
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