A tricky way to do this is to make use of DER files, and to recreate one to import your key. This is a little hacky, but it works.
This is how I did it in a few minutes:
1/ First, create a private key (you won't use, you just need the file structure) in PEM format:
$ openssl ecparam -name secp256k1 -genkey -noout > private.key
We could directly create a DER file though, using a -outform DER, by the way.
1 (optional)/ What's cool about DER format is that you can dump in ASN.1 its structure. It is interesting and you should take a look at it:
$ cat private.key | openssl asn1parse -in - -inform DER
0:d=0 hl=2 l= 116 cons: SEQUENCE
2:d=1 hl=2 l= 1 prim: INTEGER :01
5:d=1 hl=2 l= 32 prim: OCTET STRING [HEX DUMP]:BCDA55068C63324BB5CD05696264ED474467A3F1CC95CE83C00BC497F064F4B6
39:d=1 hl=2 l= 7 cons: cont [ 0 ]
41:d=2 hl=2 l= 5 prim: OBJECT :secp256k1
48:d=1 hl=2 l= 68 cons: cont [ 1 ]
50:d=2 hl=2 l= 66 prim: BIT STRING
2/ From the private.key PEM file, convert it in DER and extract the header, the key, and the footer.
$ ll private.key
-rw-------. 1 mycroft mycroft 223 Mar 23 06:51 private.key
$ openssl ec -in private.key -outform DER 2>/dev/null|head -c 7 > header.bin
$ openssl ec -in private.key -outform DER 2>/dev/null|tail -c +8|head -c 32 > key.bin
$ openssl ec -in private.key -outform DER 2>/dev/null|tail -c +40 > footer.bin
$ ll *bin
-rw-rw-r--. 1 mycroft mycroft 79 Mar 23 06:53 footer.bin
-rw-rw-r--. 1 mycroft mycroft 7 Mar 23 06:53 header.bin
-rw-rw-r--. 1 mycroft mycroft 32 Mar 23 06:53 key.bin
3/ Replace the key I just created by yours
$ echo A114dad00000000000faced00000000000bad000000000decaf00c02bad02bad | xxd -r -p > key.bin
4/ Rebuild & check the complete DER file
$ cat header.bin key.bin footer.bin > private_key.der
$ cat private_key.der | openssl asn1parse -in - -inform DER
0:d=0 hl=2 l= 116 cons: SEQUENCE
2:d=1 hl=2 l= 1 prim: INTEGER :01
5:d=1 hl=2 l= 32 prim: OCTET STRING [HEX DUMP]:A114DAD00000000000FACED00000000000BAD000000000DECAF00C02BAD02BAD
39:d=1 hl=2 l= 7 cons: cont [ 0 ]
41:d=2 hl=2 l= 5 prim: OBJECT :secp256k1
48:d=1 hl=2 l= 68 cons: cont [ 1 ]
50:d=2 hl=2 l= 66 prim: BIT STRING
See ? Same structure, different key! Voodoo magic!
5/ recreate your "Key" file:
$ openssl ec -in private_key.der -inform DER -text -noout
read EC key
Private-Key: (256 bit)
priv:
a1:14:da:d0:00:00:00:00:00:fa:ce:d0:00:00:00:
00:00:ba:d0:00:00:00:00:de:ca:f0:0c:02:ba:d0:
2b:ad
pub:
04:d5:53:99:c9:4d:1f:1f:7f:f1:5b:c5:13:d1:96:
34:2b:c8:22:dc:23:9d:f0:55:d6:72:3c:f0:b6:2c:
c8:20:e8:dd:13:40:4b:3a:0d:e5:64:ce:6a:20:e6:
23:ee:2a:17:18:23:d0:6f:0b:13:30:4a:d7:f5:2a:
c2:28:2e:40:33
ASN1 OID: secp256k1
And we're done.