I don't know if this widely known or actually useful to anyone, but I've just discovered if you:

a) copy and paste (as an Enhanced Metafile) a dashed line from Inkscape (v0.92)

or

b) import an svg file containing dashed lines from Affinity Designer (v1.6)

The dashed line styles can then be picked up and applied to new lines.

I'm not sure how this actually happens as the dashed line styles are not shown as document brushes. This means the style has to be picked up by selecting the imported line first, and the option under the General tab 'Give new objects most recent attributes' must be selected. It is however possible to copy and paste format/attributes from one line to another.

Click image for larger version. 

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