Here's the code I ended up with for copying a Matrix3f into the format for a rotation Matrix:
protected Matrix Matrix3fToMatrix(Matrix3f input)
{
//Declare variables
float[] insertDump = new float[16]; //The array holding the values in the new Matrix
float[] rowDump = new float[3]; //The array for temporarily holding values from the rows of the Matrix3f
int targetElement = 0; //Points to the next location in the insertDump to add data
//Loop through the rows of the Matrix3f
for (int sourceRow = 0; sourceRow < 3; sourceRow++)
{
//Grab the row from the Matrix3f
input.getRow(sourceRow, rowDump);
//Insert the 3 elements from the Matrix3f into the Matrix. The fourth element has been initialized to 0.0f.
insertDump[targetElement] = rowDump[0];
insertDump[targetElement + 1] = rowDump[1];
insertDump[targetElement + 2] = rowDump[2];
//Move the target ahead by four spaces
targetElement += 4;
}
//The final row consists of { 0.0f, 0.0f, 0.0f, 1.0f }, since this is a rotation matrix
insertDump[15] = 1.0f;
//Create the new Matrix and load in the values
Matrix output = new Matrix();
output.setDump(insertDump);
//Return
return output;
}