I've attached a long wire aerial to the whip on my PL-380. It has no benefit to medium or long wave reception. Actually it deteriorated the signal as suggested in Scott's post.
A solution for connecting an external antenna is in John Bryant's article on adding antenna inputs to the Tecsun receivers (in the Files section). It does required some careful surgery.
For those interested in FM DXing with a barefoot ULR, the PL-380 is ideal.
Good DX all.
36°22'51"N / 97°26'35"W
(near Perry OK USA)